26,075 research outputs found
May a dissipative environment be beneficial for quantum annealing?
We discuss the quantum annealing of the fully-connected ferromagnetic -spin model in a dissipative environment at low temperature. This model, in
the large limit, encodes in its ground state the solution to the Grover's
problem of searching in unsorted databases. In the framework of the quantum
circuit model, a quantum algorithm is known for this task, providing a
quadratic speed-up with respect to its best classical counterpart. This
improvement is not recovered in adiabatic quantum computation for an isolated
quantum processor. We analyze the same problem in the presence of a
low-temperature reservoir, using a Markovian quantum master equation in
Lindblad form, and we show that a thermal enhancement is achieved in the
presence of a zero temperature environment moderately coupled to the quantum
annealer.Comment: 4 pages, 1 figure, proceeding of IQIS 201
Consistent use of paradoxes in deriving constraints on the dynamics of physical systems and of no-go-theorems
The classical methods used by recursion theory and formal logic to block
paradoxes do not work in quantum information theory. Since quantum information
can exist as a coherent superposition of the classical ``yes'' and ``no''
states, certain tasks which are not conceivable in the classical setting can be
performed in the quantum setting. Classical logical inconsistencies do not
arise, since there exist fixed point states of the diagonalization operator. In
particular, closed timelike curves need not be eliminated in the quantum
setting, since they would not lead to any paradoxical outcome controllability.
Quantum information theory can also be subjected to the treatment of
inconsistent information in databases and expert systems. It is suggested that
any two pieces of contradicting information are stored and processed as
coherent superposition. In order to be tractable, this strategy requires
quantum computation.Comment: 10 pages, latex, no figure
Quantum Navigation and Ranking in Complex Networks
Complex networks are formal frameworks capturing the interdependencies
between the elements of large systems and databases. This formalism allows to
use network navigation methods to rank the importance that each constituent has
on the global organization of the system. A key example is Pagerank navigation
which is at the core of the most used search engine of the World Wide Web.
Inspired in this classical algorithm, we define a quantum navigation method
providing a unique ranking of the elements of a network. We analyze the
convergence of quantum navigation to the stationary rank of networks and show
that quantumness decreases the number of navigation steps before convergence.
In addition, we show that quantum navigation allows to solve degeneracies found
in classical ranks. By implementing the quantum algorithm in real networks, we
confirm these improvements and show that quantum coherence unveils new
hierarchical features about the global organization of complex systems.Comment: title changed, more real networks analyzed, version published in
scientific report
Grover's search algorithm: An optical approach
The essential operations of a quantum computer can be accomplished using
solely optical elements, with different polarization or spatial modes
representing the individual qubits. We present a simple all-optical
implementation of Grover's algorithm for efficient searching, in which a
database of four elements is searched with a single query. By `compiling' the
actual setup, we have reduced the required number of optical elements from 24
to only 12. We discuss the extension to larger databases, and the limitations
of these techniques.Comment: 6 pages, 5 figures. To appear in a special issue of the Journal of
Modern Optics -- "The Physics of Quantum Information
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